The animal group classified as being the most critically endangered is the

Amphibians

Explanation:

The most critically endangered species is that of the amphibians and they are characterized by the IUCN and are facing a very high rate of extinction in the wild. This depends on the population size with species size being reduced by 90% and occurs over less than the over less than 100 km and its number is less than 50 million and there is a 50% of the chances that the species will go extinct in the wild.

I'm pretty sure it's c. extremely gradual but not 100% sure though.

Answer were lower;